Two-way signal transmission system



1942- D. MITCHELL 2,302,374

TWO-WAY SIGNAL TRANSMISSION SYSTEM Filed Feb. 25. 1941 I a I l ll u E Qh 5 IL a a a a lNl/ENTOR D. MITCHELL waw A TTORNE V Patented Nov. 17,1942 UNITED STATE s PATENT OFFICE TWO-WAY SIGNAL TRANSMISSION SYSTEMDaren Mitchell, Bound Brook, N. .I., aim to Bell Telephone Laboratories,Incorporated, New

Claims.

The invention relates to a two-way transmission system and particularlyto a two-way signaling system including a two-wire line and an adjoininglink, such as a four-wire circuit, providing separate paths, one fortransmitting to the two-wire line and the other for transmitting fromthat line. It is well known in the art that systems of this sort areused in connecting a toll line from a distant point to a localsubsciber. The two-wire circuit to the local subscriber may andcustomarily does include one or more interoflice trunks and thesubscribers line so that the impedance of the two-wire line for a givenconnection depends upon the particular route and instrumentalities usedin extending the connection to th local subscriber.

In systems of this sort where a four-wire circult is connected to atwo-wire line, as is well known in the art, a hybrid coil is used and anartificial balancing line is provided for the purpose of balancing thetwo-wire line. As the impedance of the two-wire line may vary, as abovestated, it is difilcult to obtain an accurate balance with a givenartificial line. It has been proposed, however. to provide switchingarrangements which interchange-artificial lines and in sure in aparticular connection the use of an artificial line which is a fairapproximation to that of the two-wire line. Where unbalance is presentdue to inaccurate matching of the artificial line and the actual lineundesirable effects arise, the most important of which manifests itselfin echoes and, as is well known in the art, devices known as echosuppressors are generally provided to minimize this effect.

In accordance with the present invention it is proposed to provide anartificial balancing line which can be adjusted continuously during thepassage of the signals and in this way provide a balance sufficientlyclose so that the echo suppressors may either be left out or used in amanner such that their efiects on the transmission are less harmful thanif a fixed artificial balancing line were used. Broadly, in accordancewith the invention, devices are provided which function automatically todetermine at any instant whether the line impedance or the network im'pedance is higher and to raise or lower the network impedanceaccordingly. Also. in accordance with the invention, his control isexercised by the distant talker rather than by the local v subscriberand instrumentalities are provided whereby the talking currentsoriginated by the local talker are ineffective in exercising anyconvantage oi such arrangement is that once the balance has been arrivedat in response to the voice currents from the distant talker it will notbe disturbed when the local talker takes command oi the circuit.

In a copending application of K. G. Van Wynen, Serial No. 380,428, filedFeb. 25, 1941, other arrangements for a similar purpose are disclosed,in which an optimum artificial line is automatically selected andassociated with the two-wire line prior to the start 0! conversation.

In the drawing the single figure shows schematically a circuit diagramof an incoming toll telephone connection involving the connection of afour-wire circuit to a two-wire line.

Referring to the drawing, the four-wire toll circuit includes theoppositely-directed two-wire circuits L2 and L3 and the two-wire line isindicated by the line LI The usual amplifiers or repeaters, one for eachof the circuits L2 and L3, are indicated by the triangles A, the apex ofeach triangle indicating the direction of transmission. The two-wirecircuit Li is extended to the local subscriber's station and may includea number of links, not shown, such as interofiice trunks and cordcircuits, as is common in well-known telephone toll switching practice.The usual hybrid coil for connecting the four-wire circuit to thetwo-wire line is indicated by H and the artificial line or balancingnetwork is shown at AL.

The impedance of the artificial line AL as shown is arranged to bevaried by the operation of a well-known type of two-motion switchindicated generally at S. In switch S, a source of power M continuouslyrotates the disc El which may be frictionally engaged with either of thediscs E2 and E3 by the operation of the magnets MI and M2. As shown, thediscs E2 and E3 are flexibly mounted on the shaft SI coupled to theswitch-arm S2 which varies the impedance of the artificial line. Thearrangement is such that if the magnet MI is energized the switch-arm S2is moved in a clockwise direction to increase the trol over thebalancing arrangement. The ad- 56 a measuring relay MR, and the other adirection indicating relay DR, these relays through their armatures andcontacts serving to connect the energizing battery B to magnet Ml ormagnet M2, or to disconnect the battery from either magnet, depending onthe manner of relay operation to be described. The relay MR is adjustedso that with equal currents in its two windings its armature remains ina neutral position. It the current in the left-hand winding is greaterthan in the right-hand winding, an upper contact is made and underreverse conditions a lower contact is made. The DR relay is biased sothat the left-hand winding is rendered less sensitive than theright-hand winding and its contact is maintained closed except when thecurrent in the left-hand winding is sumcient to overcome the moresensitive right-hand winding. The diflerence in sensitiveness of the twowindings is provided by shunting a suitable resistance RI across theleft-hand winding.

Connected across the two-wire line Ll is a control circuit I includingin order the high impedance input amplifier Al, band-pass filter FI andrectifier DI the output of which is passed through the left-handwindings of the relays MR and DR in series. Connected across theartificial line AL is a second control circuit 2 including the highimpedance input amplifier A2, band-pass filter F2, and the rectifier D2,the output of which is passed through the righthand windings of therelays MR and DR in series. Smoothing condensers Cl and C2 are connectedacross the output of the rectifiers DI and D2, respectively. Theresistance RI, the function oi which has been described, being connectedin the distant party is talking, the telephonic signal currents will bereceived over circuit L3 through amplifier A and through the lowerwindings oi. the hybrid coil H. If there is an approximate balancebetween the two-wire line and the artificial line, as will normally bethe case, this energy will divide between the two-wire line LI and theartificial line AL depending upon the degree of balance. As the inputimpedances oi amplifiers Al and A2 in control circuits l and 2 are highand equal, a small portion of this energy will pass through each ofthese circuits I and 2 in which it will be amplified by the amplifier Alor A2, filtered by filter Fl or F2, rectified by rectifier DI or D2 andpassed through the lefthand or right-hand windings of the MR and DRrelays, respectively. If the impedance of the two-wire line Ll islighter than that of the artificial line Al, the voltage applied to AIwill be greater than that applied to A2. Thus, since these amplifiershave high impedance inputs, the current in the output 01' the rectifierDI, including the left-hand windings of the MR and DR relays, will begreater than the current in the output of the rectifier D2, which passesthrough the right-hand windings of the MR and DR relays. Under theseconditions, the MR relay will close its upper contact but the DR relaywill not open its contact because, as previously explained, it is biasedso that the current passing certain predetermined margin that flowing inits right-hand winding. As longas this unbalance exists between theartificial line and the twowire line, the relay MR will close its uppercon-- tact and the switch 8 will drive the switch-arm S2 in itsclockwise direction to increase the amount oi. impedance in AL. when abalance is obtained, the relay MR will restore to its neutral positionand the artificial line AL willremain in adjustment. It, on the otherhand, the energy in the control circuit 2 is measurably greater than inthe control circuit 1, the right-hand windings oi the relays MR and DRwill become eiIective and the relay MR will close its lower contact andthe switch 8 will move the switcharm S2 in the opposite direction todecrease the amount of impedance in AL until it reaches a balancedpoint.

Inasmuch as the artificial line AL should be balanced with the two-wireLl with respect prin cipally to the incoming energy, means are providedto prevent this balance from being upset by transmission in the otherdirection. In the first place, it is obvious that the local subscriber'stransmission may contain considerably more energy than the distantsubscriber's and that such means must be provided to prevent the localsubscriber from taking command of the circuit and upsetting the balance.This is accomplished by the DR relay. Energy from the local subscribersline Ll will pass through hybrid coil H into the outgoing circuit L2,and under balanced conditions a much greater portion of that energy willpass through the control circuit I from Ll than will pass through thehybrid coil H to the artificial line AL and to control circuit 2, sothat under any ordinary conditions a much greater amount oi. energy willpass through the letthand windings of relays DR and MR than will passthrough the right-hand winding of these relays. As a result, the DRrelay will immediately release to disconnect battery B so that operationof the MR relay by control circuit l or 2 will not result in theenergization of the magnets Ml or M2 from battery B. Thus, thepreviously established adJustment oi the artificial line AL will not bedisturbed by the outthrough its left-hand winding must exceed by a 76and described which are within the spirit and scope of the inventionwill be apparent to persons skilled in the art.

What is claimed is:

1. In combination in a two-way signal transmission system, a two-waysignal transmission medium of variable impedance characteristics, atour-wire signal transmission circuit comprising two oppositely directedone-way signal transmission paths, a bridge circuit comprising a hybridcoil and an associated balancing network coupling said medium to oneterminal 01' said tour-wire circuit, and means to reduce echoes in saidfour-wire circuit during signal transmission between it and said mediumcomprising measuring means for effectively balancing the portion oi.incoming signals transmitted from the incoming path 01' said four-wirecircuit through said hybrid coil to said medium against the portion ofincoming signals transmitted from said incoming path through said hybridcoil to said balancing network, and means responsive to the directionand amount of the measured unbalance to adjust the impedance of saidnetwork accordingly so that it substantially balances the impedance ofsaid line.

2. In combination in a telephone system, a two-way telephone line ofvariable impedance characteristic, a four-wire telephone circuit, ahybrid coil and associated balancing network for connecting one terminalof said four-wire circuit to said telephone line, and means to reduceechoes in said four-wire circuit during signal transmission between saidfour-wire circuit and said line comprising measuring means foreffectively balancing the portion of incoming telephone signalstransmitted from said four-wire circuit through said hybrid coil to saidline against the portion thereof transmitted from said four-wire circuitthrough said hybrid coil to said network, and means responsive to themeasured unbalance to adjust the impedance of said network accordinglyuntil it provides a tolerable balance of the impedance of said line.

3. In combination in a telephone system, a two-way subscriber'stelephone line, a four-Wire telephone circuit, a hybrid coil andassociated balancing network coupling said line and circuit, and meansto reduce echoes in said circuit during telephone transmission betweensaid line and circuit, comprising measuring means for effectivelybalancing the voice signals in said line received over saidfour-wirecircuit from a distant subscriber against the voice signalssupplied to said network, means responsive to the direction and amountof the measured unbalance to adjust the impedance of said balancingnetwork accordingly so that it substantially balances the impedance ofsaid line, and means to prevent false adjustment of the impedance ofsaid network when voice signals from. the local subscriber are beingreceived over said two-way subscriber's line.

4. In combination in a telephone system, a two-way subscribers telephoneline, a four-wire telephone circuit, a hybrid coil and associatedbalancing network coupling said line and circuit, and means to reduceechoes in said circuit during telephone transmission between said lineand circuit, comprising two voice-operated switching circuitsrespectively connected across said two-way line and said network onopposite sides of said hybrid coil, differential relay means controlledby said switching circuits, operating in one direction for a greaterpredetermined voltage input to the switching circuit connected acrosssaid line than to the switching circuit connected across said network,and in the opposite direction for a predetermined greater voltage inputto the switching circuit connected across said network than to theswitching circuit connected across said line, means responsive to theoperation of said difierential relay means in said one direction toincrease the impedance of said network and responsive to the operationof said differential relay means in said opposite direction to decreasethe impedance of said network, and means to prevent false adjustment ofthe impedance of said network in response to voice signals from a localsubscriber received over said line.

5. In combination, in a telephone system, a two-way subscriberstelephone line, a four-wire telephone circuit, a hybrid coil andassociated balancing network coupling said line and circuit, and meansto reduce echoes in said circuit during telephone transmission betweensaid line and circuit comprising one voice-operated switching circuitconnected across the line side, said hybrid coil and anothervoice-operated switching circuit connected across the network side ofsaid hybrid coil, differential relay means controlled by the twoswitching circuits'operating in one direction for a predeterminedgreater voltage input to said one switching circuit than to said otherswitching circuit and. operating in the other direction for apredetermined greater voltage input to said other switching circuit thanto said one switching circuit, means for increasing the impedance ofsaid network when said differential relay means is operated in said onedirection and decreasing its impedance when said differential relaymeans is operated in said other direction, in order to improve thebalance of said line, and other differential relay means controlled bysaid switching circuits operating to prevent the adjustment of theimpedance of said network when voice signals are being received oversaid line from the associated telephone subscriber.

